Such protective devices have become known under the name "airbag" and in the meantime are installed as standard in modern motor vehicles. Airbag modules accommodated in the steering wheel of a motor vehicle are usual and are the most widespread, which in the case of an accident are triggered by an acceleration sensor, the folded gas bag being inflated explosion-like and forming between the driver and the steering wheel a cushion by which injuries are prevented, but at least reduced. Also, corresponding airbag modules have been provided as standard in the meantime for the passenger. Further airbag modules which are matched to particular accident situations, for example to a side impact, are likewise offered as additional equipment or even already as standard.
In all airbag modules, the gas bag can only develop an optimum protective effect when it is inflated promptly in a taut manner. In the case of front airbags, promptly means for example within 20 to 30 milliseconds. To achieve this, pyrotechnic charges are generally used, which can make available the necessary gas volume as a product of chemical conversions in the manner of an explosion.
As a gas bag which is inflated in a taut manner would act like an elastically deformable spring element and would throw back the occupant who is plunging into it, in all airbag systems care must be taken that the filling gases can also flow out from the gas bag again, because only thus can a substantially plastic collision behavior be achieved with optimum decrease of kinetic energy when a vehicle occupant plunges in. For this, either the material of the gas bag casing is designed so as to be gas-permeable in a defined manner, or else with a gas bag casing which is substantially impermeable to gas, separate outflow openings are provided. In both cases, it can not be avoided that already on filling the gas bag a greater or lesser proportion of the filling gases flows off and the maximum possible filling can not be achieved with a given propellant charge. If one nevertheless wishes to achieve a particular filling level, the propellant charge must be designed to be larger so that the quantity of filling gast to be regarded as loss, which flows off until the occupant plunges, can be compensated.
From this, the problem results of improving the device initially mentioned to the effect that on the one hand a gas bag, which is filled in a taut manner, can be made available and that on the other hand no losses of filling gases occur and the filling volume which is aimed for is achieved with the smallest possible quantity of propellant.